Investigation of power conversion systems applicable to high speed and variable speed shafts have received much attention recently because of their ability to generate power at higher speeds with reduced weight and size. They are of particular interest due to recent interest in their use with windmills and flywheels. In flywheel storage systems the conversion system, and particularly the motor generator machine, must be capable in one mode of generating power at higher and variable frequencies and presenting it at constant and conventional lower frequency and in another mode of accepting power at the constant lower frequency and converting it to a higher variable frequency to drive the flywheel. When used with windmills and other prime power sources the system must be capable of converting the generated higher varying frequency electrical signals to constant lower frequency signals. See Report R-960, Interim Report on Research Toward Improved Flywheel Suspension and Energy Conversion Systems, by David Eisenhaure, George Oberbeck, Stephen O'Dea, and William Stanton.
In general, the variable speed (frequency) power must be converted to an existing fixed frequency for use. Many different combinations of conventional power conversion systems have been applied to this task, each with its own deficiencies. Typical systems have employed multiple machine configurations, variable mechanical speed reducers, A.C.- D.C.-A.C. converter-inverters, cycloconverters, field modulated down converters and many other arrangements. Many of these systems have suffered from large weight and size, inefficiency due to poor waveform quality and form factor, or unreliability due to complexity and switching losses.